new guy with a case 444

hi. i've had my case 444 for 5 years it's been a great little tractor. i lent it to a neighbor last year to clear snow and he thought he was doing me a favor by refueling. he added gas to the hydraulic tank. flushed it 3 times. lately it does'nt have much push. this is recent but it may be related to that incident.i put 20-50 in the system. today it would'nt start and i have intermittent spark at the plug. i have had to replace the plug a few times wht does it go through plugs so fast? well. enough of my rambling. thinking about trying to find a small tractor with a loader but lack the funds right now. thanks for looking. ken..